The ${}^{252}\mathrm{Cf}$ spontaneous fission (sf), prompt fission neutron spectrum (PFNS) is a fundamental quantity for nuclear physics measurements of neutron-emitting reactions. This energy distribution of neutrons emitted from fission has been considered a neutron data standard for decades and has been utilized as a reference for neutron detection efficiency, validation of Monte Carlo simulations, benchmarking of dosimetry standards, and more. A significant portion of the global collection of nuclear data on neutron-induced reactions is correlated with the ${}^{252}\mathrm{Cf}\left(\mathrm{sf}\right)$ PFNS. Despite the reliance on this quantity by the nuclear physics community, the historical collection of ${}^{252}\mathrm{Cf}$ PFNS measurements display systematic disagreements that are not understood or easily explained. These experimental discrepancies could potentially bias the ${}^{252}\mathrm{Cf}$ PFNS Standard evaluation. On top of this, these past experiments frequently employed correlated experimental measurement or analysis methods. The artificial intelligence (AI)/machine learning (ML)-informed californium chi-nuclear data experiment (AIACHNE) project was formed to (a) investigate these discrepancies utilizing AI/ML methods to identify outlying regions of literature data, assign these regions to features of the experiment itself, and perform an improved evaluation of the ${}^{252}\mathrm{Cf}$ PFNS and (b) perform a new experimental measurement of this quantity designed to improve upon the existing literature database. In this work we report on the AIACHNE ${}^{252}\mathrm{Cf}$ PFNS experiment utilizing a new analysis method uncorrelated with all previous measurements: neutron efficiency determinations based on elastic neutron scattering on ${}^{12}\mathrm{C}$ and ${}^9\mathrm{Be}$. This new method provides an independent test of the existing literature data and evaluation of the ${}^{252}\mathrm{Cf}\left(\mathrm{sf}\right)$ PFNS. The method is described with detailed covariance quantification procedures, as well as a direct discussion of the sources of uncertainty described as requirements in the “Templates” series of papers. The ${}^{252}\mathrm{Cf}\left(\mathrm{sf}\right)$ PFNS reported in this work agrees well with the overall shape of the existing standard PFNS evaluation as well as many literature measurements, thus verifying the current evaluation utilizing new techniques. However, the results suggest that there are deficiencies in the angle-differential ${}^{12}\mathrm{C}$ and ${}^9\mathrm{Be}(n,n)$ evaluated nuclear data, which produce unphysical structures in the reported result. While these structures are relatively minor, they become obvious because of the high statistical precision of the data and the expected smooth continuity of the ${}^{252}\mathrm{Cf}\left(\mathrm{sf}\right)$ PFNS.

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• Fission
• Neutron physics
• Nucleon induced nuclear reactions
• A ≥ 220
• Spectrometers & spectroscopic techniques